From the first-floor window of the flat where he was incarcerated, 15-year-old Tung began to piece together what the UK was like. He liked watching the busy road with three or four shops, a pizza restaurant and a petrol station. He had been told never to turn on the light, so he often sat by the window in the dark, peering out from the side. “Where I lived in Vietnam was a very remote area, just trees and dirt tracks. We rarely saw a car. I found it all so surprising.”

He was locked in the flat alone for two months. “It was terrible, the first month. I wanted to go out, to talk to someone. I almost felt like I was going mad. But by the second month I was getting used to it.”

For several years, Tung was one of Britain’s cannabis slaves: a hidden group of exploited children, locked up alone and forced to tend plants in converted houses, in dangerous conditions. Like many other trafficked child cannabis gardeners, when the flat was raided, he was sent to prison.

The story of how much of the UK’s cannabis is grown sounds too far-fetched to be true: an international network of traffickers brings teenage boys from Vietnam to become enslaved gardeners in British suburbs. Yet every few weeks, another farm is discovered and new arrests are made.

Local newspapers offer fleeting insights into the industry, with scant facts: that police raided a red-brick terrace house on a suburban road in Liverpool after neighbours complained of the smell, and found the house stripped of furniture and converted into a cannabis farm, with two teenage boys hiding, terrified, beneath the floorboards. That a cannabis gardener in County Armagh was found living on tins of dog food. Or that police visited a two-storey house in Plymouth and found cannabis plants in every room, cellar to attic, tended by a Vietnamese boy with injuries to his face, who said he was 13; he was put in the care of social services while investigations were launched, but within days he had disappeared.

Usually, the teenagers who have been forced into this work are too anxious about retaliation from traffickers to speak. But two child cannabis slaves agreed to talk to the Guardian on the condition that their real names were not printed; both remain terrified of the organised gangs who controlled them.

Bao, 15 (photographed to protect his identity), was told: ‘If you don’t water properly, we will stop bringing you food.’ Photograph: David Levene for the Guardian

Bao was 15 when he was brought to a three-roomed flat on the edge of a large English city and left alone with hundreds of cannabis seedlings. “When I arrived, I was told that from now on I would have to take care of the plants,” he says, remembering the day when he was driven by a Vietnamese man and woman to the small second-floor flat, already filled with black plastic pots containing small cannabis plants. He had no idea what they were or why he had to care for them.

There was enough food in the freezer to last him a month, and he was told that if he did a good job, he would be brought a new supply of food. In the same breath, they warned him: “If you don’t do the watering properly, we will stop bringing you food and you will starve.”

He was given precise instructions on how to manage the complex lighting system, so that the flat’s electricity supply did not overload. “I had to be very careful that the electricity did not burn anything or explode.” A sofa had been squeezed into the narrow corridor for him to sleep on, to avoid wasting space that could be used for plants.

Most of the time, Bao was alone, sometimes for as long as three weeks, with no human contact. Occasionally, two men would visit to inspect the plants, to assess whether the shoots were thriving. This was a moment of high stress. “They poked the earth in the pots to see if it was dry. I felt scared when they checked. They said if it was not good enough for them, they would beat me up.”

Last year, police raided a former bank in Grimsby, a disused sports centre in Wales and an empty GP surgery in Harlow

Watering the plants each morning took between two and three hours. Then there was nothing to do until the evening, when at 10pm Bao would take another three hours to water the plants again. There was no television and he felt very alone. Every couple of days, he would get a phone call from the Vietnamese woman who had shown him what to do, asking precise questions about the rate of growth and giving him instructions. The windows were covered, so he couldn’t see out, but he could hear the sound of passing aeroplanes. “Sometimes I could hear the noise of the pub opposite. When I heard people laughing, having fun with their friends, I felt sad and lonely. I would play Candy Crush on my phone to distract myself,” he says. This went on for about five months. “Of course I felt lonely, but that was normal for me. I’ve been lonely all my life.”

Before he was kidnapped and smuggled into the UK, Bao, who was orphaned as a baby, was homeless and sheltering under a bridge with other children. Two years after his rescue, he is beginning to talk about his experiences. But he still finds it so painful that he has to stop periodically, going for air outside the interview room at the Children’s Society, the charity that has supported him.

Unusually, Bao was not locked inside, but by the time he was taken to the flat, he was so ground down and confused that he did not consider attempting to escape. He spoke no English and was unable to read the signs on the shops; in any case, he had no money. Occasionally, he found the heat or the heavy, sweet smell of the cannabis plants so overpowering that he left the house to walk around the block. “I didn’t think about running away. I thought, ‘There are no good or kind people around, so it is better to stay here,’” he says, picking at his nails, uncomfortable at being asked to describe how he felt.

While he is happy to talk about the experience of being a cannabis gardener, and wants people to know what it was like, he does not want to describe his life before he was brought to the suburban flat. Instead, he asks his case worker, James Simmonds-Read, to explain. It is a deeply disturbing story of 15 years of unhappiness.

There were 4,000 plants at a cannabis factory discovered by police at this former leisure centre in Newport, Gwent. Photograph: Wales News Service

Both Bao’s parents, fishermen in a rural part of Vietnam, were killed in a car crash when he was a baby, leaving him to be brought up by his grandparents, who lived in a wooden shack by a river. By the time he was 10, both of them had died, and he was living on the streets of a city in Vietnam, selling lottery tickets to feed himself.

When he was about 14, he was kidnapped by two men while he slept under the bridge; he was bound and gagged using duct tape, put into a sack and then into the boot of a car. Some time later, he was taken to China, where for several months he worked and slept in a warehouse, packaging saucepans. He was malnourished and beaten if he made the mistake of speaking while working. Later again, a group of young workers were taken and put into a cold shipping container, given a bag of bread and a bottle of water each, and kept there for around three months, while the ship travelled to a country that might have been France. He was later driven to the UK, smuggled above the wheels of a lorry.

The truck stopped in a forest in the UK, where he was shuffled into another vehicle and taken to a house. He was kept there for 10 months and forced into sex work. Then, for reasons that are no clearer to him than any of the other abrupt changes, he was driven to a house somewhere quiet, left alone and told to tend to the plants.

People said, ‘Why bother? It’s only cannabis.’ This was hard manual labour. You can see how bad the exploitation is

Living in a flat that has been converted into a cannabis farm is fraught with danger. “Above my head there were wires hanging down,” Bao says, “and I had to be careful to make sure the duvet didn’t catch fire. There were wires everywhere, powering all the electricity to the room. I had to step around them when I was watering, and they were hung quite low – so if I wasn’t careful, it would burn my hair. That happened quite a few times. Sometimes, I would brush past the lights and singe my hands and arms. I found it tiring. There were so many plants to look after, and the flat was very squashed.”

Bao was under strict instructions not to answer the door to anyone, so when police knocked five months after he had arrived at the flat, he did not answer; instead, they knocked the door down. He tried to hide beneath the cannabis plants that had flourished under his care and grown to waist height. But the police found him and bombarded him with questions, which, not speaking a word of English, he didn’t understand. He was handcuffed, taken away and held in police custody overnight. A solicitor was found, who advised him to plead guilty to cannabis cultivation, regardless of the fact that he was clearly a child and had been trafficked.

Simmonds-Read, who works for a specialist service supporting boys who have been trafficked to the UK, along with the child-trafficking organisation Ecpat UK, has found that boys are being held for years at a time. “Recently, I’ve been coming across people who have been held for longer and longer periods. I’ve met children who have been held for four or five years. It is very, very rare that they are able to escape.” Sometimes, children have food posted through the letterbox. “The vast majority were homeless orphans in Vietnam. Often cultivating cannabis is the least bad experience in their lives.”

Bao is now at college, filling in the gaps in his education, which ended when he was 10. He is hoping to raise awareness of a hidden phenomenon. “When I see people smoking cannabis, I feel that they are exploiting children like me. I want people to know that people suffer in the process of producing cannabis.”

For the past decade, this crime has been hidden in plain sight, with criminal gangs hedging their bets by setting up multiple cannabis factories in terrace houses around the country, the crops spread between houses, each relatively expendable if a single farm is discovered. But recently the police have noticed a trend towards bigger sites, often run by British criminals, using Vietnamese traffickers to provide the gardeners.

Two boys and a Vietnamese man in his 30s were arrested by the police when they raided the building; a third teenager was found the following day, wandering the rural lanes near the Wiltshire village of Tisbury. He had apparently escaped by sawing his way through a metal ventilation tunnel when the detectives arrived at 3am. He had no money and spoke no English, and was very scared when police found him. In this case, the police stated that all four of the bunker’s inhabitants were believed to be victims and did not press charges. They are currently in an immigration detention centre. Three other men (two British and a third with a Vietnamese surname) were arrested and charged with slavery offences, stealing electricity and growing cannabis.

The room where the Vietnamese boys slept was originally designated as a sick bay, for the government officials who never lived there. When I visited the site last month, I found the breeze-block room half-heartedly decorated with two Vietnamese calendars (2016 and 2017) hanging from the ceiling, a golden Buddha, piles of Vietnamese DVDs, four mattresses on the floor and summer clothes hanging on a wire strung across the room.

The entrance to the nuclear bunker in Wiltshire, raided by police in February. Photograph: David Levene for the Guardian

Whoever was supplying the food had an eye for the welfare of the workers, and had brought in vitamin tablets, Lemsip, tangerines, and boxes of onions and ginger for the men to cook Vietnamese food in the dilapidated military kitchen, where the tiles are crumbling from the walls. The deep freezers were full of supplies. There was a punchbag for the teenagers to unleash frustration and several antique, broken Pac-Man machines.

The plumbing didn’t work well, and there were 28 five-litre plastic bottles filled with urine in the toilets. In the corridors, there were several Asda shopping trolleys for pushing soil from one room to the next. Each room had around 25 plugs wired precariously to an electricity supply, allegedly stolen from the mains; an electrician who advised the police after the site was discovered said the wires were hot and dangerous, and might have caused a fire at any moment.

The smell was so bad, it made me want to throw up. It was hard to breathe. It was hard to sleep. It damaged my lungs

Some of the windowless rooms (marked “male dormitory” and “female dormitory”, “government departments” and “scientists”) had industrial plant food piled high, dozens of bottles of Canna Rhizotonic root stimulator stacked up to the ceiling, and hundreds of pots of Grotek Monster Bloom fertiliser. Conditions would have been tropical when the lights were running; police said that when they arrived, the walls were wet with condensation. Yellow hoses snaked through the corridors into 20 rooms, and huddles of bamboo sticks were resting in corners, along with head-torches and boxes of black latex gloves. The washing-up had been done tidily and the corridors were swept clean of soil.

The sweet, heavy smell of several thousand cannabis plants must have added to the sense of claustrophobia; after three hours in the dark tunnels with the police and a photographer, I was relieved to leave, to smell the cold February air and hear birdsong.

Police are trying to establish how long the Vietnamese teenagers had been there, and how long the operation had continued. Detective inspector Paul Franklin, who leads a team specialising in drug crime in Wiltshire, says that when the force posted news of the raid on its Facebook page, many commenters thought a nuclear bunker filled with cannabis was the coolest thing ever. “People said, ‘Why are you bothering? It’s only cannabis,’” Franklin says. “They’re not seeing this set-up. This was hard manual labour. You can see how bad the exploitation is.” Franklin adds that, over the past decade, Vietnamese organised criminals have linked up with gangs in the UK to supply a niche service of slave labour. Teenage girls are trafficked into prostitution and to staff nail bars; boys are brought in to grow cannabis. So much cannabis is grown in the UK that it is now a net exporter of the drug, Franklin says. “Cannabis doesn’t feature highly in terms of police priorities; heroin and crack cocaine do. If you can grow cannabis, in terrace houses, under the radar, we probably don’t hit that many of them and there is still good money to be made from it. Perhaps there’s an argument that if it were legalised, you could do it upfront and you wouldn’t all need this. But that’s for the government to decide.”

***

In January, Helen Jenkins got a tremendous shock when the police called to tell her they had raided a house she owned in Plymouth and discovered a cannabis farm. There they found a Vietnamese boy with injuries to his face, who said he was 13. Jenkins had no idea: she had let it to a “delightful” woman for three years with no problems, but in October it was sublet. That person installed one or possibly more gardeners inside, instructing them to keep the curtains closed around the clock. Jenkins knew nothing of it until the police got in touch.

“It was an absolute nightmare,” she says on the telephone, still audibly shocked three months on. “The house was wrecked. It was filthy – there was soil everywhere, under the floorboards. There was a huge hole in the ceiling with big ventilation ducts coming down. I was horrified. The worst was when you went upstairs: all the ceilings were hanging down from the weight of the soil in the loft. I’m so lucky it’s a very good, strong Victorian house: any modern house would not have withstood it.”

By the time Jenkins got there, police had removed the cannabis plants, leaving 400 pots full of soil. The freezer was full of food, suggesting that the boy had been left with supplies to keep him going for some time.

“They had nailed up the back gate, so no one could get in,” Jenkins says. “There was foil all over the walls, there were loads of UV lamps, the doors had disappeared or were wrecked. It smelled quite nice, I thought – a sweet, pleasant sort of smell. I haven’t really had anything to do with cannabis before. I was surprised no one rang me to say there was something dodgy going on. The first estimates from the builders said it was going to cost £20,000 to repair. It has nearly ruined me.”

According to the National Police Chiefs Council’s lead on cannabis-related crime, Bill Jephson, drugs from smaller farms like this one would probably be sold by local drug dealing-networks, while larger crops are divided up and sold around the country.

The police released very little information about what happened after they arrested the teenager. Local newspapers reported that he had gone missing shortly after being put into foster care, which is not unusual. Kent (which has most child migrants because of its Channel ports) loses large numbers of Vietnamese children every year from care. Most owe money to traffickers and know their families are at risk if they are not compliant, so they flee from social services back to their traffickers and are quickly put back to work.

***

Like Bao, Tung was also forced into cannabis cultivation and prostitution. He left his home in rural Vietnam when he was 15, and arrived in the UK in the summer of 2010. His mother had paid £10,000 to send him to the UK to join his father, who had left because he couldn’t make ends meet as a farmer. Tung found out later that she couldn’t afford the full payment to the people trafficker, and so had sold the family home. He travelled to the UK in a lorry, via Russia, and eventually ended up in a terrace house in a town whose name he still doesn’t know, where he was shown into a flat above a shop that had been converted into a cannabis farm.

He soon developed an allergic reaction to the smell of the cannabis and the strong chemicals that were used in its cultivation. His skin became very itchy. When we meet at the Palm Cove Society in Yorkshire, part of a rehabilitation programme run by the Salvation Army, his cough is so bad, he occasionally finds it hard to speak.

“The smell was so bad, it made me want to throw up,” he tells me. “It was hard to breathe. It was hard to sleep. It damaged my lungs.” Tung is speaking through a translator because, after six-and-a-half years in England, so much of it spent in slave-like conditions, he has had little exposure to the outside world and his English remains poor. “I wasn’t scared. I felt more sad than anything. I felt alone and I was worried about my family. I thought I should try to do this work and pay off the debt as soon as possible.”

There was nothing for him to do; there was a television, but it didn’t work. Most of the windows were covered with sheets to stop neighbours seeing what was going on inside and help prevent the smell from escaping; very occasionally, he spoke to his mother. He didn’t tell her he was locked in a flat, because he didn’t want to make her upset.

It was hard work and in very hot conditions – the flat was kept between 30C and 40C – and he wore shorts and a T-shirt, even in winter. Every three or four days, a man would arrive with a bit of food. “He would just say, ‘Are you OK?’ I would say, ‘OK.’ He would look around the flat, check everything was OK and leave.”

When Tung became unbearably unhappy, he would lie on the floor and put the duvet over his head. “I had a lot of dreams when I was there. I thought about my family. I wanted to be able to go to school and have a normal life in the UK.”

One night, the flat was raided by police and he was taken away. He didn’t understand why he was being arrested and had no idea he was growing illegal drugs; later, he found out a thermal-imaging helicopter had detected the heat from the grow lamps. He was taken to a foster family, but the next night called his traffickers, who came and took him away. “I can see now that it was a mistake to call them, but I was still hoping to find my father.”

A police officer inside the cannabis farm in the Wiltshire bunker. Photograph: David Levene for the Guardian

For the next few years, Tung was forced to do odd jobs, helping to set up other cannabis houses all over the country, driving from Wales to Scotland, and often living in the van. He was told that the debts to his traffickers had spiralled to £100,000, so at night he was made to work as a prostitute. “I didn’t want to; I escaped, but they found me, beat me and told me bad things would happen to my mum and dad. I was taken from place to place, sometimes to small hotels or houses or even shops, wherever the customer arranged for me to come. There were men and women. I was paid about £100 a month. I didn’t dare to ask about the debt, because every time I asked, I was beaten up.”

Later, he was made to work in another cannabis factory behind a bakery, and was caught when police raided it. By that point, he was 20, and was initially sentenced to three years in prison. This was reduced to 12 months, because he pleaded guilty in the police station. He spent eight months after that in an immigration detention centre. He lost contact with his parents and now worries that his sister may have been taken away by people traffickers.

“I had no idea about being a victim or about slavery until I had an interview with someone from the Home Office for an asylum application,” Tung says. Looking back, he thinks it is an accurate description: “Yes, I felt I was a slave.”

There are no accurate figures on the numbers of young people trafficked from Vietnam to do this work, but the woman who is translating for Tung estimates she has translated in “hundreds” of court cases where young people have given similar accounts. Tung thinks that during his time in the UK he must have met more than 100 people from Vietnam made to do this kind of work. “The youngest was 14,” he says. “I felt sorry for them. I worry about the fact that more young Vietnamese people are being brought here.”

Despite repeated promises to tackle this crime, and a visit by David Cameron to Vietnam in 2015, when the issue was raised, there has never been a prosecution of a Vietnamese people trafficker in the UK. According to Philippa Southwell, a lawyer who is currently representing more than 50 Vietnamese victims of human trafficking, there are many Vietnamese young people still in prison for cannabis offences, despite having been forced into criminality. “I am busier than ever,” she says. “There are prosecutions on a daily basis. It is alarming that we are still prosecuting victims of trafficking.”

Tung has agreed to talk about his experience now because he wants people in Vietnam to have a greater understanding of the dangers of being smuggled to the UK. “Anyone who is told to borrow money from someone to come here, with the promise of a better life, don’t listen,” he warns. “They will be made to do things they never wanted to do. I am worried about the possibility of being deported back to Vietnam, where I will be found by the gangs again and retrafficked. I feel very scared.”

• If you suspect someone to be a victim of modern slavery in need of assistance, or consider yourself a victim, you can call the Salvation Army’s 24-hour confidential referral helpline on 0300 303 8151.